1. Field of the Invention
The present invention relates to a transformer capable of achieving a compact structure by increasing a coil occupancy within a wiring frame without increasing a production cost, and more particularly to a transformer utilizing a prepreg tape for insulation.
2. Discussion of the Background
In a transformer employed in a power source such as a switching regulator, an insulation structure in prior technology is generally classified into following three types, which are suitably selected according to the application or the required performance.
A first insulation structure is a tape insulation structure. In this structure, two coils respectively wound circumferentially on a bobbin are mutually insulated by an insulation tape which is likewise wound circumferentially on the bobbin.
A second insulation structure is based on a divided bobbin. In such structure, a flange is provided also within a spool of a bobbin, and the coils are mutually insulated by being separately wound on spool portions separated by the flange. A third insulation structure is based on resin molding. In this structure, two coils are mutually insulated by filling a gap of the two coils, constituting different poles, with a resin.
In the first insulation structure, however, though the adjacent coils of different poles are mutually insulated by the insulation tape, a barrier tape or the like has to be provided on end faces of the coils in order to secure a creepage distance or a spatial distance required for safety standards, thus resulting in a portion where the coils cannot be provided. As a result, the occupancy ratio of the coils within the spool becomes lower, leading to a larger dimension of the transformer.
Also the second insulation structure based on the divided bobbin requires a portion where the coils cannot be provided as in the case of the first insulation structure, whereby the occupancy ratio of the coils within the spool becomes lower, leading to a larger dimension of the transformer.
On the other hand, the third insulation structure based on the resin molding is not associated with the drawback of the larger dimension of the transformer as in the first or second insulation structure, but it requires a special equipment in the production process of the transformer and also requires a long time for the resin curing, thus involving an increased production cost.
On the other hand, a prepreg tape is employed in a structure described in JP-A No. 9-330826 described above, but such structure is associated with a drawback of being in capable of securing a creepage distance or a spatial distance between a primary coil and a secondary coil or between a higher voltage coil and a lower voltage coil required for the safety standards.